4.6.1 Approved Fire Resistance Ratings
4.6.1.1 Listings
Most countries require that fire resistance tests be certified by a recognized testing laboratory or approval agency. In North America, independent testing organizations such as Underwriters Laboratories (UL, 2012) and Southwest Research Institute (SWRI, 2012) maintain registers of approved assemblies to which they have assigned fire resistance ratings. Most of these ratings are based on tests which they have carried out in accordance with recognized testing stan- dards. Generic ratings based on these approvals are listed in national building codes (e.g.
NBCC, 2010; ABCB, 2015; ICC, 2015). Some trade organizations (e.g. Gypsum Association, 2012; ASFP, 2014) maintain industry listings of approvals for products manufactured or used by their members.
Listings generally fall into three categories: generic ratings; proprietary ratings; or calcu- lation methods.
4.6.1.2 Generic Ratings
Generic fire resistance ratings, or ‘tabulated ratings’ are listings which assign fire resistance to typical building elements with no reference to individual manufacturers or detailed specifica- tions. For example, many national codes list tables of generic ratings for fire protection of structural steel members by encasement in a certain thickness of concrete, with no details of concrete quality or reinforcing. Generic ratings are derived from many full‐scale fire resis- tance tests carried out over many years. Generic ratings are widely used because they can be applied to commonly available materials in any country. Generic ratings are usually very conservative, and they are often inadequate because they apply only to standard fire exposure, and make no allowance for the size and shape of the fire‐exposed member or the level of load.
4.6.1.3 Proprietary Ratings
Proprietary fire resistance ratings apply to proprietary materials or structural products made by specific manufacturers. Proprietary ratings are based on the results of full‐scale fire tests commissioned by the manufacturers. Proprietary ratings are usually accompanied by an approved specification detailing the materials and construction methods, and it is the assem- bly rather than the materials which has the approved rating. Unless they are covered by a suitable agreement, proprietary ratings cannot be applied to similar products from other man- ufacturers because there may be differences in materials or installation methods, and the fire resistance rating may legally be the property of the manufacturer.
Proprietary ratings may be less conservative than generic ratings because they relate to more closely defined products. Proprietary ratings are usually based on standard fire exposure and make no allowance for the level of applied load, but they sometimes include reference to the size and shape of the fire exposed member in a more accurate way than generic ratings.
4.6.1.4 Calculation Methods
As the art and science of fire engineering develops, it is becoming more feasible to assess fire resistance by calculation as well as by test. Many listing agencies and national design codes now include approved calculation methods for assessing fire resistance. Many of these methods are described in this book. Calculation methods should be verified by full‐scale fire resistance test results of similar assemblies.
4.6.1.5 Expert Opinion
Most of the listings described in the above documents are based directly on the results of full‐scale fire resistance tests. Such fire tests are very expensive, so testing and approving authorities are increasingly asked to give written expert opinions on assemblies which are similar but different to those which have passed a test. An increasing number of listed fire resistance ratings are based on such expert opinions. The opinion will state whether the assem- bly would be considered likely to pass a test, based on observations of similar successful tests and the considered experience of the testing and approving personnel.
As an indication of the factors to be considered in making an opinion, Figure 4.9 illustrates a useful set of empirical ‘rules’ for comparing fire resistance of similar assemblies (Harmathy, 1965). These ‘rules’ of fire endurance have stood the test of time and are applicable in almost all situations. These rules have been expanded and explained in more detail by Lie (1992).
4.6.2 Fire Resistance by Calculation
Figure 4.10 shows a flow chart for the process of calculating the strength of a structural assem- bly exposed to a complete burnout of a fire compartment. The resulting load capacity can be compared with the expected applied load at the time of a fire, to verify whether the design is satisfactory. This is design in the strength domain. The process of calculating structural fire behaviour has three essential component models: a fire model; a heat transfer model; and a structural model.
4.6.2.1 Fire Model
Fire models have been discussed in Chapter 3. Input can be any selected time–temperature curve including the standard fire, a measured real fire or a parametric fire curve.
4.6.2.2 Heat Transfer Model
The heat transfer model is an essential component of calculating fire resistance because the load capacity or the containment ability of a fire exposed element or structure depends on the internal temperatures. The temperature of any material exposed to a fire increases as heat is conducted from the hot fire exposed surface to the cooler interior. The temperature gradients depend on the radiative and convective heat transfer coefficients at the surface, and the conduction of heat within the member. For non‐load‐bearing elements designed to contain
Rule 5
Rule 9 Rule 10
Rule 6 Rule 7 Rule 8
t2≈ t1
Beam tested as part of floor
Beam tested separately
For a beam when tested separately For the floor
assembly
A B
Beam A can be replaced by Beam B if t2> t1 t1> t2
t1> t2
t1> t2 t1≠ t2
t1 t2
t1
t1
t2
t2
t1 t2 t1 t2 t1 t2
Fire
Low conductivity High conductivity Low conductivity High conductivity Moist DryFireFire Fire
Figure 4.9 Harmathy’s ten rules of fire endurance. Reprinted with permissionfrom Harmathy (1965).
© 1965 National Fire Protection Association, all rights reserved
fires, the output from a heat transfer model can be used directly to assess whether the time to critical temperature rise on the unexposed face is acceptable. For simple structural elements with a single limiting temperature, the output from a heat transfer model can be used directly to assess whether the critical temperature is exceeded. These situations do not require the application of a structural model. They are examples of verification in the temperature domain.
For more complicated structural elements or assemblies, the output from the heat transfer model is essential input to a structural model for calculating load‐bearing capacity. Temperature gradients within a member may or may not be significant. When a material such as steel with a high thermal conductivity is heated slowly, as in a protected assembly, it may be sufficiently accurate to disregard temperature gradients and assume that all the material is at the same temperature. For materials with low thermal conductivity like concrete, it becomes very important to know the thermal gradients during the fire because these affect the temperature of the reinforcing steel. Heat transfer calculations are less important for large timber members
Activity
Room geometry
Element geometry Thermal properties
Element geometry Applied loads Mechanical properties Heat transfer coefficients
Fuel load Fire characteristics
Construction
FIRE MODEL
Fire thermal exposure
HEAT TRANSFER MODEL
Thermal gradients
STRUCTURAL MODEL
Load capacity
Figure 4.10 Flow chart for calculating strength of a structure exposed to fire
4.6.2.3 Structural Model
Models for calculating the performance of structural elements exposed to fire are described in Chapter 5. Hand calculation methods can be used for simple elements but sophisticated com- puter models are necessary for the analysis of frames or larger structures. Computer‐based structural analysis models must be able to include the effects of thermal expansion, loading and unloading, large deformations and non‐linear material properties which are temperature‐
dependent, all for a framework of interconnected members of different materials. Hand calcu- lation methods for the main structural materials are given later in this book.